This invention relates to effects in nonlinear optical materials and, more particularly, to phase matched second harmonic generation.
A number of nonlinear optical materials, have large optical nonlinearities, but either are optically isotropic or have only a small birefringence so that collinear optical beams cannot be phase matched. In this category of materials are cubic semiconductors such as ZnS, GaAs and GaP. Several methods have been proposed which introduce an effective optical anisotropy and are capable of cancelling the effect of natural dispersion. N. Bloembergen et al, Applied Physics Letters, Vol. 17, p. 483 (1970), proposed a periodic structure comprising many alternating layers of two semiconductors and utilized the change in the dispersion curve for optical radiation propagating perpendicular to the layers. An alternative proposal by Y. Yacoby et al, J. Applied Physics, Vol. 44, p. 3180 (1973), utilized a multilayered medium in which the sign of the nonlinearity alternated every coherence length.
A second category of prior art devices involves phase matching in a waveguide in which waveguide dispersion produces an effective anistropy for different modes. See D. B. Anderson et al, Applied Physics Letters, Vol. 19, p. 266 (1971) and U.S. Pat. No. 3,537,020 issued on Oct. 27, 1970.
The class of prior art devices which utilizes a multilayered structure with the radiation propagating perpendicular to the layers suffers from the need for a large number of such layers (often a hundred or more) in which thickness and composition must be critically controlled. On the other hand, prior art devices incorporating a waveguide pay a penalty in a large reduction in the effective nonlinear constant whenever the fundamental and second harmonic fields do not have the same mode order.